Cup-shaped or tray-shaped plastic containers are used for filling of various different foods and beverages such as prepared food products, kneaded fish products, frozen foods, cooked foods, rice cakes, liquid soups, seasonings, drinking water and the like, and then have cover materials heat sealed to flange sections to produce packaged semi-processed products in various forms, after which the packaged semi-processed products are subjected to retort sterilizing treatment at, for example, approximately 110° C. to 130° C. and approximately 1-3 Kgf/cm2·G for about 20 to 60 minutes, to produce retort sterilizing packaged foods in various forms.
The cover materials used to seal the openings of such containers must not undergo interlayer separation (delamination) by retort sterilizing treatment, and must also exhibit sealing performance that reliably protects the quality of the contents while also being easily openable, i.e. having an easy-peeling property, to facilitate opening at the time of consumption. In addition, bonding between the cover material and the container is usually carried out by heat sealing, but from the viewpoint of cost and productivity it must exhibit excellent seal strength by brief heat treatment at low temperature.
Conventionally, the cover materials used for containers for retort sterilization have been laminates composed of two layers, such as an aluminum foil layer and a sealing layer. The laminate must be subjected to high-temperature, prolonged heat sealing of about 190° C.·1 second for bonding with the container, and therefore cost and time are necessary for production, while they have also had problems of inferior physical strength such as pierce resistance and tearing strength.
On the other hand, packaging materials composing containers for retort sterilization have been disclosed in PTLs 1 and 2, for example, as multilayer laminated films having the following structure: 12 μm-thick polyethylene terephthalate film/7 μm-thick aluminum foil/15 μm-thick nylon film/50 μm-thick sealing layer. (Throughout the present specification, references to laminar structures of laminated bodies with slashes (/) indicate adjacency to the surrounding layers.) The multilayer laminated films described in PTLs 1 and 2 have excellent pierce resistance and tear resistance, but have still required high-temperature, prolonged heat sealing of about 190° C.·1 second for bonding of the multilayer laminated films to containers.